Abstract : Thornel 300 Graphite/S-Glass Rovings/Narmco 5208 epoxy hybrid composites were investigated to establish the effect of temperature and moisture on the fatigue, impact and residual Cafter cycling) mechanical properties. The preconditioning treatments were high humidity (98% at l20 deg F) coupled with and without thermal shocks, The stress cycling was accomplished at 75 deg F, 98% RH and phi = 1800 cpm. The results showed that hybrid graphite/glass/epoxy composites can be manufactured with properties at least as go as the high modulus all-graphite/epoxy composites and at considerably reduced costs over an all-graphite composite. Overall, the hybrid composites not only can produce cost effective composites but actually can possess mechanical properties considerably improved over the single phase systems. A comparison of the all glass systems and the hybrid glass/graphite system shows that the transverse fatigue strengths are higher for the all-glass composite, where the interfacial bonding is expected to be good, than for the hybrid system where some graphite/epoxy interfacial bonds are present. The 1000 hour high moisture exposures on these two systems, however, result in a very similar residual fatigue strength This is taken to indicate that the principal degradatory mechanism of moisture is on the interface between the fiber and the matrix and that the graphite composite degradation will be no worse than that for glass composites. (MM)